Abstract

Being able to track animal movement and interactions in their natural habitat is fundamental to understanding their ecology and evolution. And yet, tracking the behavior of freely roaming and untagged animals in natural environments is notoriously challenging. Weakly electric fish with their continuously active, individual-specific electric organ discharge (EOD) are ideal subjects for monitoring undisturbed individual behavior. We here present automated, EOD-based tracking methods that allow for discerning presence and movement of multiple fish simultaneously. Applying our technology for monitoring a multispecies community of wave-type weakly-electric fish in their natural habitat in Darien, Panama (Apteronotus rostratus, Eigenmannia humboldtii, and Sternopygus dariensis), we found consistent upstream movements in all three species after sunset followed by downstream movements in the second half of the night. Extrapolating these movements and using estimates of fish density obtained from additional transect data, we hypothesize that some fish might cover at least several hundreds of meters of the stream per night. Most fish, including Eigenmannia, were traversing the electrode array solitarily. From in-situ measurements of the decay of the EOD amplitude with distance of individual animals we estimated that fish can detect conspecifics at a distance of up to approximately 1.7,m for Apteronotus and 2.5,m for the two other species. Our recordings also emphasize interactions of the electric fields between the three species resulting in complex electrosensory scenes. EOD-based tracking provides an unprecedented window into the so-far hidden life history of weakly electric fish and makes it possible to study their behaviors at an unmatched level of detail.

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